Solid-State Synthesis and Phase Transitions in the RE<sub>2</sub>(MoO<sub>4</sub>)<sub>3</sub> Family Monitored by Thermodiffraction
Iván da Silva, Gerardo Gil de Cos, C. González-Silgo, J. López‐Solano, A. Mújica, Nivaria R. Ramírez-Rodríguez, M.E. Torres
Abstract
High Resolution Image Download MS PowerPoint Slide Solid-state synthesis and phase transitions of RE 2 (MoO 4 ) 3 (RE ≡ Nd, Sm, Eu, and Gd) samples have been monitored by X-ray thermodiffraction with synchrotron radiation. The experiment was divided in two stages. In the first heating, different non-stoichiometric molybdates (Eu 4 Mo 7 O 27, Eu 2 Mo 4 O 15, and Pr 2 Mo 4 O 15 structure types) emerged from the RE 2 O 3 and MoO 3 oxides before the expected phases (with α-Eu 2 (WO 4 ) 3 and La 2 (MoO 4 ) 3 structure types and the β-Gd 2 (MoO 4 ) 3 phase). The formation and coexistence of intermediate phases have been explained by common structural motifs with unit cell volumes per atom among those with the formula RE 2 (MoO 4 ) 3 . Subsequent heating–cooling cycles showed the occurrence of the reversible and reconstructive α [La 2 (MoO 4 ) 3 ] ↔ β phase transition, including the less common transition β → α [La 2 (MoO 4 ) 3 ] obtained by heating the β′-Gd 2 (MoO 4 ) 3 phase from room temperature and clarifying much of the controversy in the literature. The transition mechanisms were studied by proposing a common supercell and comparing the RE and vacancy ordering within similar layers of MoO 4 2– tetrahedra. The possible formation of stacking faults in Nd 2 (MoO 4 ) 3 was explained as a mixture of modulated scheelite phases. This research supports the importance of a directed and rational synthesis analyzing the intermediate products and their phase transitions for the enrichment of materials with new or improved properties.